When I first picked up some Greyhawk sourcebooks for Advanced Dungeons and Dragons I was hooked. 

The images of heraldry, Griffons, Dragons, wizards.. and castles.

One of my earliest drives as a player and Dungeon master was, running or developing characters, so they could spend their gold on a Castle! Or Inn, or Blacksmith shop..

This was for me, the reason to adventure. So you could build!

Sadly I feel in official content much of this is missing now.

Magic Item prices, ( I understand it's personal, but a basic price helps so much). 

I love how easy it is to homebrew whatever I want on D&D Beyond.

I could easily find a guide on DM Guild. 

But as an old, grumpy, full-time working D&D player I just want some nice rules in a flash book that help me keep new players interested once they killed a few things and got some gold..

That rather depends on the players too. Some won't be interested in building anything. Others will come up with stuff they want in their bespoke hangout that 1E didn't cover either. Those pesky players never want to do what I want them to do... :p

There are 3rd party systems that bring back the detail of the old DMG, such as Strongholds and Followers by Matt Coleville. Not everyone loves that book, but it does provide ways to build and upgrade castles, temples, thieves' guilds, and the like.

Alternatively, you could get a PDF of the old DMG and just use those rules. You'll probably have to convert the gp values but once you figure out the conversion rate it'd be fairly straightforward.

I'm a great fan of The Sane Magical Prices Guide, you can download the PDF if you want it.  Their prices are admittedly kind of high sometimes, but they give a pretty good run-down on why.  That might help with the magic item prices at least.  Other folks have made good suggestions.  Interesting topic, thanks for posting it.

I think if you go to level 20, the goals are far beyond building a castle. They are becoming gods, or slaying gods. Maybe you will build a castle along the way. Or maybe that's your goal if you only play to level 15.

Would you really want to think that the castle you've captured and now occupy is a generic, module-written castle that loads of other players are in? Or would you rather have a unique, bespoke built castle, or one you liberated from a vampire on an adventure?

The more you put into the game, the more you'll get out of it.

Quote from Sanvael >>

Would you really want to think that the castle you've captured and now occupy is a generic, module-written castle that loads of other players are in? Or would you rather have a unique, bespoke built castle, or one you liberated from a vampire on an adventure?

The more you put into the game, the more you'll get out of it.

Yes, but here’s the thing: if the features you want are really unique, what are the odds you can find them detailed in a book?

I will second Strongholds and Followers as a nice source for building bases of operations. At the least, it has a lot of stuff that can provide inspiration for what you want to do.

Ultimately this game is about adventuring, so I don't begrudge 5e for not having a lot of rules on settling down and running your very own Stirge Nursery or whatever. But it is useful to have this stuff as a money sink so that treasure is still relevant and sought after at higher levels.

Quote from pangurjan >>

Quote from Sanvael >>

Would you really want to think that the castle you've captured and now occupy is a generic, module-written castle that loads of other players are in? Or would you rather have a unique, bespoke built castle, or one you liberated from a vampire on an adventure?

The more you put into the game, the more you'll get out of it.

Yes, but here’s the thing: if the features you want are really unique, what are the odds you can find them detailed in a book?

 Like kotath said, some costs could be nice.

If I remember right, becmi had prices for things like 10 linear feet of one-foot thick stone wall. Similar prices for wood, or the cost to dig a moat, build a watchtower, etc. Then you let people design their own and do the math to figure out what it costs. It’s not like there’s much to spend gold on this edition — unless your DM allows magic shops. I mean, for an experienced DM, it’s probably not too hard to figure something out, but it would be nice to have an idea that’s in line with what passes for a 5e economy.

You want prices?

Diocletian's Edict on Maximum Prices

This lists prices of produce, various laborers, and raw materials in the late Roman Empire. It uses the silver denarius. You could either assume that's equivalent to 1 D&D silver coin, or maybe you can calibrate it based on the costs listed in the SRD.

Thanks!

Its more about economics, with the likely hood of lots of wealth by, 10th level, yeah castle, then higher levels, Kingdom, lvl 20 Empire. It usually works well. As I said, magic, weapons, are all like buying a car. Does the job, doesn't create an economy. Why become rich? unless you want more? Why fight an Orc if all you dream off is a bed of straw and some rat stew that cost you half a copper..

Makes Gold a reward and shows how it influences the world. Not just a number and encumbrance value.

Awesome thanks!

I am starting the party off with the option of Managing an Inn. The Player characters are at 6th level now (if they want, it's an option for them as they now have gold, and mounts in training).

Castles and Strongholds give Inn the ability to "gather intelligence" wich works well with the campaign I am running. They can also invest gold into the Inn and get income from the Inn.

This to me is the start of being an entrepreneur adventurer. Gives meaning and goal to someone to go and adventure! Make more gold and affect the world they play in!. 

Dungeons and Financial Administrators is a different game to Dungeons and Dragons.

You will find that asking players to design a castle whilst calculating the cost of a 10 foot length of 5 foot wide stone wall is not only going to lose their interest real quick, but it's also irrelevant. Everything comes down to an estimation in the end. You're far better off doing broad calculations and giving the players a figure so that they can go out and focus on the interesting parts of the game - exploring, solving mysteries, killing monsters - rather than sitting making Excel spreadsheets to see what the optimum design for the bell tower is, and trying to source cheaper iron from the elves for the bell vs. dwarven mass produced bells. If you and your group think designing things on this micro level is really fun, then you have to recognise that you have a niche interest and try to enjoy the homebrewing.

Bear in mind that The Tower of London took 20 years to build. Notre Dame cathedral took nearly 200 years to build. If you are aiming for your PCs to have epic fortifications and the like, it better be a long, long campaign in which case all you need is an annual spend. Not all castles were so slow to build, but if you want castles, you're far better off having the PCs storm one and liberate it from a bad guy and letting them make a few additions. Once they're in possession of lands and titles, they will also have an income of tithes that you get to decide, making the cost of doing anything simply a time based choice that you make (e.g. it costs 20,000 gold to add a new wing to the caste; you have given the PCs taxes of 5,000gp per years).  

The cost of these things is going to be entirely dependent on the world you create, and the amount of money the PCs find, so a generic price list isn't that helpful.

Thanks, I enjoyed the Roman values a lot. The game I am running is based in a more feudal area of Forgotten Realms. So giving things like Land as payment is a thing. Long-term campaign in a focused kingdom area. So far my Pc's are just hoarding gold. So i'm trying to dangle out motivations for them to spend it. Otherwise gold becomes sand and no one cares about it fast..

I find having an Inn or farm, or castle, allows players keep playing outside the table adventures. So if we can't all get together to adventure, we can do some work, spend some gold on other things that give some bonuses in the table top game. Downtime activities, you can only craft so many swords, or make so much hide armor untill that gets old..

Quote from joshakawa >>

Thanks, I enjoyed the Roman values a lot. The game I am running is based in a more feudal area of Forgotten Realms. So giving things like Land as payment is a thing. Long-term campaign in a focused kingdom area. So far my Pc's are just hoarding gold. So i'm trying to dangle out motivations for them to spend it. Otherwise gold becomes sand and no one cares about it fast..

 

 Maybe hint that thieves operate in the area. They can rob your gold, but it's much harder to rob a feudal title.